Optical communication systems are known in which an optical transmitter transmits an optical signal on an optical communication path to a downstream optical receiver. The optical signal is often modulated to carry information to the receiver that is often spaced from the transmitter by tens or hundreds of kilometers. The optical communication path typically includes one or more segments of optical fiber and various nodes, such as optical amplifier nodes and reconfigurable optical add/drop (ROADM) nodes.
The performance of the optical transmitter may be monitored by detecting various parameters, such as the optical signal-to-noise ratio (OSNR) and bit error rate (BER) of the optical signal. When these parameters, for example, are at desired levels, the performance of the optical transmitter may be considered optimized. If the optical transmitter does not transmit at an acceptable baseline performance level and the BER and OSNR are not at minimally acceptable levels, for example, during system start-up, tuning of the wavelength of light output from the optical transmitter, optimal performance may be difficult to adjust to the optimal level based on optical signals monitored at a distant receiver. In particular, when monitoring the optical signal at a remote receiver, impairments due to transmission through the optical communication path, e.g., through fiber, optical amplifiers and OADMs, may be difficult to distinguish from impairments due to faults associated with the optical transmitter itself. Distinguishing between the two classes of impairments is further made difficult in systems including photonic integrated circuits that supply optical signals having different wavelengths. Identifying the source of impairments is also difficult in systems in which the optical signals are modulated in accordance with advanced modulation formats, such as quadrature phase shift-keying (QPSK).
So-called intradyne coherent optical systems are known that include receivers having known digital signal processing circuits that operate in accordance with known algorithms to demodulate a received signal, apply error correction, and determine various characteristics of the optical signal. In order to monitor to thereby adjust parameters of an optical transmitter in such systems for optimal performance, a dedicated receiver may be provided locally, near the transmitter, to thereby directly monitor the optical signals prior to transmission along the optical communication path. As a result, so-called back-to-back BER measurements, for example, can be obtained directly proximate to the output of the optical transmitter, and such measurements are free of distortions or impairments related to the optical communication path.
Although accurate measurements indicative of the performance of the transmitter itself may be obtained with a dedicated receiver, the cost, power consumption and physical space required for the dedicated receiver are often prohibitive. Accordingly, there is a need for an efficient and inexpensive method and apparatus for locally monitoring the performance of an optical transmitter, such as an optical transmitter in a coherent optical communication system.